The new legislation follows an outbreak of measles that started at Disneyland and sickened 147 people in the U.S. and Mexico. The Centers for Disease Control and Prevention and California state health officials said pockets of people philosophically opposed to vaccines were helping measles cases imported from other countries take hold and spread.

States that let parents opt out for philosophical reasons have higher rates of unvaccinated kids than Mississippi and West Virginia, the only two other states that allow only medical exemptions.

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The United States is one of the few countries where states and school districts require proof of vaccination for children to attend public schools. There is no legal requirement to vaccinate children. Parents can home-school unvaccinated children.

Polls suggest most Americans do support and believe in vaccines. A Pew Research poll found that 83 percent of the U.S. public agrees that vaccines for diseases such as measles, mumps and rubella (MMR) are safe for healthy children. The survey found that 92 percent of college graduates agree that vaccines are safe but this drops to 77 percent of people with a high school degree or less.

Nationally, about 91 percent of kids get the combined measles, mumps and rubella vaccine before they start preschool.

Now, 19 states allow philosophical exemptions, according to the National Conference of State Legislatures. All the rest except for California, Mississippi and West Virginia, allow religious exemptions.

The Illinois legislature has sent a new bill to the governor that would tighten exemptions,requiring parents or guardians who want to claim a religious exemption to detail their objections for specific immunizations, with a doctor or nurse’s signature, and to file for each child before kindergarten, sixth and ninth grade.